1. Field of the Invention
The present invention is concerned with a method and an apparatus for the thermal control of an electric motor fitted in a vehicle, the electric motor being part of an electro-mechanical actuating system of the vehicle. It is especially applicable to a power assisted steering system.
2. Description of the Prior Art
Actuating systems mounted in vehicles are subjected to temperature environments which vary from one zone to another within the same vehicle.
In addition, it is known that electric motors dissipate energy due to the Joule effect in their windings, especially when they are under mechanical load. In the extreme case, when the rotor of the motor is stopped by too large a mechanical load, the motor, supplied with unidirectional current, behaves largely like a resistor.
Electric motors are very often equipped with thermal decoupling devices which detect when the safe temperature of the motor is exceeded by too large an amount while it is operating. Such devices include for example thermocouple devices which interrupt the supply to the electric motor, for example.
However, these devices, necessary though they may be, do not detect environmental variations truthfully, and in addition they work in a static manner. In particular, they only detect relatively long-term mean values.
In addition, the opening of the relay that connects the electric motor to its power supply is controlled in an abrupt manner. This feature is a drawback when it is desired to avoid any shock in the drive via the actuating system in which the electric motor is installed. Thus, when the actuating system operates in a continuous mode, the thermal control systems of the prior art introduce a discontinuity which is detrimental to the actuating system.
Furthermore, when the actuating systems are fitted in a vehicle, cooling of the electric motor takes place substantially exponentially, with a characteristic cooling time constant. In this connection, when the driver of the vehicle in which the electric motor is fitted interrupts the general power supply by turning the ignition key, the electric motor, being connected to the general power supply of the vehicle and being thus interrupted, cools down slowly. In a typical example, it passes from its nominal operating temperature (150.degree. in one example) to the ambient temperature (20.degree.) in about thirty minutes. If the vehicle is restarted, or there is a call for use of the electric motor, before this period of return to ambient temperature has elapsed, the thermal control of the motor does not start from a predetermined initial state.
This is also the case when an electronic controller calls for the power supply to the actuating system to be stopped.
In a power assisted steering system, depending on driving situations and/or the attitudes of the vehicle, the calls for changes of direction of the steerable road wheels are interpreted by .an electronic computer which controls the electrical supply of a motor that serves as a mechanical energy source. If the thermal load on the electric motor reaches too high a value, a conventional thermal protector, such as a thermocouple, causes the power assistance for the steering to be interrupted suddenly. As a result, the driver would receive a shock on the steering column which would be dangerous.